1. Field of the Invention
This invention relates to the rotor position detection of polyphase electrical machines, and, particularly, but not exclusively, polyphase switched reluctance machines.
2. Description of Related Art
The characteristics and operation of switched reluctance systems are well known in the art and are described in, for example, “The characteristics, design and application of switched reluctance motors and drives” by Stephenson and Blake, PCIM'93, Nürnberg, 21-24 June 1993, incorporated herein by reference. FIG. 1 shows a typical switched reluctance drive in schematic form, where the switched reluctance motor 12 drives a load 19. The input DC power supply 11 can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive, and a rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor.
Many different power converter topologies are known, several of which are discussed in the Stephenson paper cited above. One of the most common configurations is shown for a single phase of a polyphase system in FIG. 2, in which the phase winding 16 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the “DC link” of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A resistor 28 is connected in series with the lower switch 22 to provide a current feedback signal. A capacitor 25, known as the “DC link capacitor”, is connected across the DC link to source or sink any alternating component of the DC link current (ie the so-called “ripple current”) which cannot be drawn from or returned to the supply. In practical terms, the capacitor 25 may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter. A polyphase system typically uses several “phase legs” of FIG. 2 connected in parallel to energize the phases of the electrical machine.
FIG. 3 shows typical waveforms for an operating cycle of the circuit shown in FIG. 2. FIG. 3(a) shows the voltage being applied for the duration of the conduction angle θc when the switches 21 and 22 are closed. FIG. 3(b) shows the current in the phase winding 16 rising to a peak and then falling slightly. At the end of the conduction period, the switches are opened and the current transfers to the diodes, placing the inverted link voltage across the winding and hence forcing down the flux and the current to zero. At zero current, the diodes cease to conduct and the circuit is inactive until the start of a subsequent conduction period. The current on the DC link reverses when the switches are opened, as shown in FIG. 3(c), and the returned current represents energy being returned to the supply. The shape of the current waveform varies depending on the operating point of the machine and on the switching strategy adopted. As is well-known and described in, for example, the Stephenson paper cited above, switching off the switches non-simultaneously gives an operating mode generally known as “freewheeling”, in which the current circulates around a loop comprising the winding, one switch and one diode. This technique is used for various reasons, including peak current limitation and acoustic noise reduction.
At zero and low speeds, however, the single-pulse mode is not suitable, due to the high peak currents which would be experienced, and the chopping mode is used. As for single-pulse control, there are two principal variants of the chopping mode. The simplest method is simultaneously to open the two switches associated with a phase winding, e.g. switches 21 and 22 in FIG. 2. This causes energy to be returned from the machine to the DC link. This is sometimes known as “hard chopping”. The alternative method is to open only one of the switches and allow freewheeling to occur: this is known as “freewheel chopping” or “soft chopping”. In this mode of control, no energy is returned to the DC link from the phase winding.
With any chopping scheme, there is a choice of strategy for determining the current levels to be used. Many such strategies are known in the art. One commonly used scheme is to use a hysteresis controller which enables chopping between upper and lower currents. A typical scheme is shown in FIG. 4(a) for hard chopping. At a chosen switch-on angle θon (which is often the position at which the phase has minimum inductance, but may be some other position), the voltage is applied to the phase winding and the phase current is allowed to rise until it reaches the upper hysteresis current Iu. At this point both switches are opened and the current falls until it reaches the lower current Il and the switches are closed again, repeating the chopping cycle. FIG. 5(a) shows the corresponding phase current waveform for a hysteresis controller using freewheeling: the reduction in chopping frequency is immediately evident.
The supply currents flowing in the DC link due to the phase currents in FIGS. 4(a) and 5(a) are shown in FIGS. 4(b) and 5(b) respectively. In each case, the DC link capacitor supplies a proportion of the ac component of these waveforms. It will be understood by the skilled person that these figures are idealized, since the capacitor must have zero mean current. In practice, the behavior of the currents in the presence of supply impedance and capacitor resistance and inductance is considerably more complex.
The phase inductance cycle of a switched reluctance machine is the period of the variation of inductance for the, or each, phase, for example between maxima when the rotor poles and the relevant respective stator poles are fully aligned. An idealized form of the inductance curve for a phase is shown in FIG. 4(a). In practice, the sharp corners would be rounded due to flux fringing and to saturation of the magnetic circuits, and the maximum value of inductance would also be current dependent. Nevertheless, this curve is useful to illustrate the general behavior of the machine.
The performance of a switched reluctance machine depends, in part, on the accurate timing of phase energization with respect to rotor position. Detection of rotor position is conventionally achieved by using a transducer 15, shown schematically in FIG. 1, such as a rotating toothed disk mounted on the machine rotor, which co-operates with an optical or magnetic sensor mounted on the stator. A pulse train indicative of rotor position relative to the stator is generated and supplied to control circuitry, allowing accurate phase energization. This system is simple and works well in many applications. However, the rotor position transducer increases the overall cost of assembly, adds extra electrical connections to the machine and is, therefore, a potential source of unreliability.
Various methods for dispensing with the rotor position transducer have been proposed, several of which are reviewed in “Sensorless Methods for Determining the Rotor Position of Switched Reluctance Motors” by W F Ray and I H Al-Bahadly, published in the Proceedings of The European Power Electronics Conference, Brighton, UK, 13-16 Sep. 1993, Vol. 6, pp 7-13, incorporated herein by reference. In general, the methods fall into two categories: those suitable for low-speed operation and those suitable for high-speed operation.
Where “chopping” current control is the dominant control strategy for varying the developed torque, known methods usually employ diagnostic energization pulses in non-torque-productive phases (i.e. those phases which are not energized directly from the power supply at a particular moment). For example, a method suited to low-speed, chopping mode is that proposed by N M Mvungi and J M Stephenson in “Accurate Sensorless Rotor Position Detection in an S R Motor”, published in Proceedings of the European Power Electronics Conference, Firenze, Italy, 1991, Vol. 1, pp 390-393, incorporated herein by reference. Such methods work best at relatively low speeds, where the length of time taken up by a diagnostic pulse is small compared to the overall cycle time of an inductance period. As speed rises, the pulse occupies a longer part of the cycle and soon the point is reached where reliable position information is not available.
In the prior art, the diagnostic pulse technique is generally applied to systems where only one phase is energized at a time. Though this is the conventional, and simplest, method of operation, it does not necessarily provide the highest specific output from the machine. In an attempt to produce the highest possible output, some advanced systems use two or more phases conducting together. In 2-phase systems it is conventional to operate the phases alternately. However, U.S. Pat. No. 5,747,962, commonly assigned to the present assignee and incorporated herein by reference, discloses a method of operating both phases simultaneously over part of the electrical cycle of the machine. In 3-phase machines, it is possible to operate by exciting Phase A alone, then Phase B alone, then Phase C alone. However, to improve both the minimum instantaneous torque and the average torque output of the machine, advantage is often taken of the fact that the torque-productive portions of each phase cycle overlap. Thus, an excitation pattern of A, AB, B, BC, C, CA, A . . . is known to be used. Similarly for 4-phase machines, there are normally always two phases producing torque in the required direction, so phases can be energized in pairs: AB, BC, CD, DA, AB . . . Corresponding rules apply for higher phase numbers, in which it is possible to use three or more phases for at least part of the electrical cycle.
Though such schemes increase the available output of the machine, they have an adverse effect on any sensorless position detection scheme using diagnostic pulses, since the magnetic circuit of the machine is now carrying flux from at least two active phases in addition to the flux associated with the diagnostic pulse in a third phase. Because the magnetic circuit is generally non-linear in its flux/current relationship, the information gleaned from the diagnostic pulse is distorted and this leads to an error in the estimation of rotor position.
There is therefore a need for a reliable method of rotor position detection in, for example, the low-speed chopping mode when more than one phase is being energized over a pre-defined rotor angle.